Array Power Supply-Based Screening of Static Random Access Memory Cells for Bias Temperature Instability

ABSTRACT

A method of screening complementary metal-oxide-semiconductor CMOS integrated circuits, such as integrated circuits including CMOS static random access memory (SRAM) cells, for transistors susceptible to transistor characteristic shifts over operating time. For the example of SRAM cells formed of cross-coupled CMOS inverters, separate ground voltage levels can be applied to the source nodes of the driver transistors, or separate power supply voltage levels can be applied to the source nodes of the load transistors (or both). Asymmetric bias voltages applied to the transistors in this manner will reduce the transistor drive current, and can thus mimic the effects of bias temperature instability (BTI). Cells that are vulnerable to threshold voltage shift over time can thus be identified.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/440,769 filed on Apr. 5, 2012 which claims priority, under 35 U.S.C.§119(e), of Provisional Application No. 61/510,788, filed Jul. 22, 2011,all of which are incorporated herein by this reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of solid-state memory. Embodiments ofthis invention are more specifically directed to the manufacture andtesting of static random access memories (SRAMs).

Many modern electronic devices and systems now include substantialcomputational capability for controlling and managing a wide range offunctions and useful applications. Considering the large amount ofdigital data often involved in performing the complex functions of thesemodern devices, significant solid-state memory capacity is now commonlyimplemented in the electronic circuitry for these systems. Static randomaccess memory (SRAM) has become the memory technology of choice for muchof the solid-state data storage requirements in these modernpower-conscious electronic systems. As is fundamental in the art, SRAMcells store contents “statically”, in that the stored data state remainslatched in each cell so long as power is applied to the memory; this isin contrast to “dynamic” RAM (“DRAM”), in which the data must beperiodically refreshed in order to be retained.

Advances in semiconductor technology in recent years have enabled theshrinking of minimum device feature sizes (e.g., MOS transistor gates)into the sub-micron range. This miniaturization is especially beneficialwhen applied to memory arrays, because of the large proportion of theoverall chip area often devoted to on-chip memories. As a result,significant memory resources are now often integrated as embedded memoryinto larger-scale integrated circuits, such as microprocessors, digitalsignal processors, and “system-on-a-chip” integrated circuits. However,this physical scaling of device sizes raises significant issues,especially in connection with embedded SRAM but also in SRAM realized as“stand-alone” memory integrated circuit devices. Several of these issuesare due to increased variability in the electrical characteristics oftransistors formed at these extremely small feature sizes. Thisvariability in characteristics has been observed to increase thelikelihood of read and write functional failures, on a cell-by-cellbasis. Sensitivity to device variability is especially high in thosememories that are at or near their circuit design limits. Thecombination of increased device variability with the larger number ofmemory cells (and thus transistors) within an integrated circuit rendersa high likelihood that one or more cells cannot be read or written asexpected.

Bias temperature instability (BTI) transistor degradation mechanismshave recently become observable at the extremely small minimum featuresizes in modern integrated circuits. One such mechanism is negative biastemperature instability (“NBTI”), which appears as an increase inthreshold voltage over time, primarily in p-channel MOS transistors. Thephysical mechanism underlying NBTI is the trapping of charge at the gatedielectric interface that occurs over time in p-channel MOS transistorsthat are biased to an “on” state (i.e., a negative voltage at thetransistor gate relative to its channel region). Conversely, positivebias-temperature instability (“PBTI”) is a similar degradation effectthat primarily affects n-channel MOS transistors biased to an “on” state(i.e., a positive voltage at the transistor gate relative to its channelregion). For MOS transistors with silicon dioxide gate dielectrics, onlyslight PBTI degradation of n-channel transistors has been observed incomparison to NBTI degradation of p-channel transistors in the samecircuits.

Recently, however, the continuing demand for ever-smaller devicegeometries has led to the more widespread use of high-k gate dielectricfilms (i.e., gate dielectric materials with a high dielectric constantrelative to that of silicon dioxide). These high-k gate dielectricfilms, which enable the formation of thicker gate dielectrics withexcellent gate characteristics, are typically used in conjunction withmetal gate electrodes, rather than polysilicon gates, due to sucheffects as polysilicon depletion. A common high-k dielectric film usedin the art is hafnium oxide (HfO₂). Examples of the metal gate materialin modern device technologies include titanium nitride (TiN),tantalum-silicon-nitride (Ta_(x)Si_(y)N), and tantalum carbide(TaC_(x)). These high-k metal gate n-channel MOS transistors have beenobserved to be vulnerable to threshold voltage shifts due to PBTI, eventhough their conventional gate dielectric n-channel devices are not.This vulnerability is believed due to the affinity of HfO₂ films to trapelectrons under positive gate bias (relative to the transistor channelregion). As in the case of NBTI, the effect of PBTI on high-k metal gaten-channel MOS transistors is an increase in threshold voltage over time.

In the context of CMOS SRAMs, BTI degradation affects the ability ofmemory cells to retain data, and to be written and read. Thesedegradation effects will be described in connection with an example of aconventional SRAM cell as shown in FIG. 1. In this example, SRAM cell 2is a conventional six-transistor (6-T) static memory cell 2, which inthis case is in the j^(th) row and k^(th) column of a memory array. SRAMmemory cell 2 is biased between the voltage on power supply line V_(dda)and a ground reference voltage V_(ssa). SRAM memory cell 2 isconstructed in the conventional manner as a pair of cross-coupled CMOSinverters, one inverter of series-connected p-channel load transistor 3a and n-channel driver transistor 4 a, and the other inverter ofseries-connected p-channel load transistor 3 b and n-channel transistor4 b; the gates of the transistors in each inverter are connectedtogether and to the common drain node of the transistors in the otherinverter, in the usual manner. The common drain node of transistors 3 a,4 a constitutes storage node SNT, and the common drain node oftransistors 3 b, 4 b constitutes storage node SNB, in this example.N-channel pass-gate transistor 5 a has its source/drain path connectedbetween storage node SNT and bit line BLT_(k) for the k^(th) column, andn-channel pass-gate transistor 5 b has its source/drain path connectedbetween storage node SNB and bit line BLB_(k). The gates of pass-gatetransistors 5 a, 5 b are driven by word line WL_(j) for this j^(th) rowin which cell 2 resides.

In its normal operation, bit lines BLT_(k), BLB_(k) are typicallyprecharged by precharge circuitry 7 to a high voltage V_(ddp) (which isat or near power supply voltage V_(dda)) and are equalized to thatvoltage; precharge circuitry 7 then releases bit lines BLT_(k), BLB_(k).To access cell 2 for a read operation, word line WL_(j) is thenenergized, turning on pass-gate transistors 5 a, 5 b, and connectingstorage nodes SNT, SNB to the then-floating bit lines BLT_(k), BLB_(k),respectively. The differential voltage developed on bit lines BLT_(k),BLB_(k) is then sensed and amplified by a sense amplifier. In a writeoperation, typical modern SRAM memories include write circuitry thatpulls one of the then-floating bit lines BLT_(k), BLB_(k) low (i.e., toa voltage at or near ground voltage V_(ssa)), depending on the datastate to be written. Upon word line WL_(j) then being energized, the lowlevel bit line BLT_(k) or BLB_(k) will pull down its associated storagenode SNT, SNB, causing the cross-coupled inverters of addressed cell 2to latch in the desired state.

BTI degradation can cause operational failures in SRAM cells that arealready vulnerable due to variability and mismatch of sub-micron minimumfeature size transistors, and other factors. In the conventional cell ofFIG. 1, NBTI can affect p-channel load transistors 3 a, 3 b, while PBTIaffects n-channel driver transistors 4 a, 4 b and n-channel passtransistors 5 a, 5 b. Typically, BTI appears at those transistors thatare biased “on” for long periods of time, such as transistors 3 b, 4 a(biased on to retain a “0” data state of storage node SNT low andstorage node SNB high) or transistors 3 a, 4 b (biased on to retain a“1” data state of storage node SNB low and storage node SNT high). Whilepass transistors 5 a, 5 b are also vulnerable to PBTI, the duty cycle atwhich these devices are biased on is much lower than for the invertertransistors. Both NBTI and PBTI are reflected by increases in transistorthreshold voltage over operating life, which manifests as cell failuresin later operating life.

One type of failure that can be caused by BTI is a read stabilityfailure, also referred to as a “disturb” failure or as insufficientstatic noise margin, in which noise appearing as an elevated voltage(e.g., 0.2 volts) at the low storage node causes a false change of stateof the cell. More specifically, this mechanism occurs in “half-selected”cells (cells in unselected columns of the selected row), upon the passtransistor passing the precharged bit line voltage to the low sidestorage node. If the low side driver transistor is not able to hold asufficiently low voltage at the storage node, this noise can be ofsufficient magnitude to trip the inverters of the cell. Read stabilityfailures can occur in cases in which the drive of the SRAM cell driveror load transistors is mismatched relative to other transistors in thecell. For the example in which cell 2 of FIG. 1 is storing a “0” state(storage node SNT low and storage node SNB high) and has been storingthis state for a long period of time, a positive voltage will have beenpresent at the gate of driver transistor 4 a over that time potentiallycausing a threshold voltage shift due to PBTI. If the threshold voltageof driver transistor 4 a has increased due to PBTI, it will haveweakened drive relative to its pass transistor 5 a, which changes theoperating point of the voltage divider of transistors 4 a, 5 a whentransistor 5 a is turned on during an access to row j. The voltage atstorage node SNT during an access to a cell 2 in row j will thus shiftto a higher than optimal voltage. This higher voltage will tend to turnon driver transistor 4 b, which would flip the state of cell 2.

Conversely, a long-held “0” data state of cell 2 can cause NBTIdegradation at load transistor 3 b, increasing its threshold voltagerelative to that of load transistor 3 a. The resulting degradation indrive strength of load transistor 3 b will reduce its ability to holdstorage node SNB to a high voltage during a noise event, which alsodecreases the static noise margin of cell 2 and increases the likelihoodof an undesired change of state.

As discussed above, a read of cell 2 is performed by energizing wordline WL_(j) to turn on pass transistors 5 a, 5 b, and sensing which ofprecharged bit lines BLT_(k), BLB_(k) are pulled down by the drivertransistor 4 a, 4 b current in its “on” state. Similarly, weakening ofthe drive of one of driver transistors 4 a, 4 b, and of one of loadtransistors 3 a, 3 b, due to PBTI and NBTI, respectively, results inweaker read current during a read cycle. Sufficiently weak read currentwill, of course, causes an insufficient differential signal to bedeveloped across bit lines BLT_(k), BLB_(k), leading to a so-called“read failure” (an incorrect data state being read). Weakened drive inn-channel pass transistors 5 a, 5 b due to PBTI can exacerbate thisweakness in read current.

SRAM cells that exhibit PBTI and NBTI are vulnerable to a similarfailure mechanism, referred to in the art as a retention stabilityfailure. This failure is manifest by the cell being unable to retain itsstored data state at a reduced power supply voltage level. As known inthe art, many SRAM memories are expected to provide the user with a lowpower “retention mode” in which the power supply voltage applied to thememory array is reduced (during which time the memory is not availablefor immediate access). The reduced power supply voltage of coursereduces the standby power consumed by the memory. As such, the abilityof the cells in the memory array to retain their stored data states inretention mode is of importance. Indeed, the retention performance ofthe weakest cell in the array effectively determines the lowest powersupply voltage available in retention mode, and thus the extent to whichpower consumption can be reduced in this mode. Weakened drive capabilitydue to PBTI in one of the driver transistors, or weakened drivecapability due to NBTI on one of the load transistors, contributes topoorer retention capability of an SRAM cell because of the resultingweakness with which the levels at the corresponding storage nodes areheld by those devices.

Another failure mechanism that can result from PBTI and NBTI degradationis a write failure, which occurs when an addressed SRAM cell does notchange its stored state in response to a write of the opposite datastate from that stored. Write failures are the converse of readstability failures—while a read stability failure occurs if a cellchanges its state too easily, a write failure occurs if a cell is toostubborn in changing its state, specifically by the write circuitrybeing unable to pull down the storage node that is currently latched toa high voltage.

For example, if cell 2 of FIG. 1 is storing a “0” data state, a highlogic level will be present at storage node SNB. If pass transistor 5 bhas degraded due to PBTI, its drive current will have weakened and thuswill reduce the ability of the low-side bit line BLB_(k) to overcome thedrive of load transistor 3 b to write the opposite “1” data state. Inaddition, if driver transistor 4 b has weakened due to PBTI, the effectof feedback from storage node SNT being pulled high by load transistor 3a during this write cycle will be reduced, further reducing thewriteability of cell 2. NBTI degradation at load transistor 3 a willalso be reflected in a potential write failure (i.e., a write from “0”to “1”) by reducing its ability to pull storage node SNT high inresponse to storage node SNB being pulled low by bit line BLB_(k).

In each case, it is contemplated that the memory cells most vulnerableto the effects of PBTI or NBTI degradation are those cells that alreadyhave a device mismatch or other asymmetry in their manufacture. Asmentioned above, such mismatches and asymmetries are more pronouncedgiven the increased variability in the electrical characteristicsobserved for transistors having extremely small feature sizes,particularly in memories that are designed at or near their circuitdesign limits.

The increased level of reliability required of modern integratedcircuits has necessitated the use of time-zero screens to remove (orrepair, by way of redundant memory cells and circuit functions) thosedevices that are vulnerable to failure over the expected operating lifeof the device. In the sub-micron CMOS SRAM context, manufacturing testflows now commonly include screens to identify or replace those memorycells that are close to a pass/fail threshold at manufacture, within amargin corresponding to the expected PBTI or NBTI drift over the desiredoperating life. A conventional approach in such screening is to apply“guardbands” on certain applied voltages during functional or parametrictests of circuit functions. In many cases, guardbanded voltages areimplemented to account for the temperature dependence of circuitbehavior, to enable the manufacturer to perform functional testing atone temperature (preferably room temperature) with confidence that thecircuit will perform according to specification over the full specifiedtemperature range, over the expected operating life. As known in theart, it is becoming increasingly difficult to design the appropriatetest “vectors” (i.e., combinations of bias and internal circuitvoltages, and other test conditions) that identify devices that arevulnerable to failure over time and temperature, without significantyield loss of devices that would not fail over operating life yet failthe screen at the applied guardbanded test vectors.

Copending U.S. application Ser. No. 13/189,675, filed Jul. 25, 2011,commonly assigned herewith and incorporated herein by reference,describes a screening method for testing solid-state memories for theeffects of long-term shift due to NBTI in combination with randomtelegraph noise (RTN), in the context of SRAM cells As described in thatapplication, each memory cell in the array is functionally tested with abias voltage (e.g., the cell power supply voltage) at a first guardbandthat is sufficient to account for worst case long-term shift and RTNeffects. Cells failing the first guardband test are then repeatedlytested with the bias voltage at a second guardband that is less severethan the first; those previously failed cells that pass this secondguardband are considered to not be vulnerable to RTN effects. Thisapproach avoids the over-screening of conventional test methods thatapply an unduly severe guardband, while still identifying vulnerablememory cells in the population for repair or as failed devices.

By way of further background, it is known in the art to apply a voltagehigher than the power supply voltage to the body nodes of the p-channelload transistors during the test of SRAM arrays. This condition isreferred to in the art as a “reverse back-bias” condition, and istypically applied to the n-well regions in which the load transistorsare formed. As fundamental in the art, this reverse back-bias voltagehas the effect of increasing the threshold voltage of the loadtransistors, and thus reducing their source-drain drive at a givensource-drain voltage and gate-source voltage. Such a test is performedwith the intent of screening out cells that are vulnerable to increasedthreshold voltage over operating time caused by NBTI.

It has been discovered, in connection with this invention, that it isdifficult to derive an accurate time-zero screen to identify thosememory cells for which NBTI and PBTI degradation will cause read orwrite failures or read stability failures. To the extent that potentialproxies for this effect are available, those proxies necessitate anexcessively harsh screen margin (i.e., guardband) to meet modernreliability goals. The undue yield loss of devices that fail such ascreen but would, in fact, not have degraded to failure, can besubstantial.

BRIEF SUMMARY OF THE INVENTION

Embodiments of this invention provide a solid-state static random accessmemory (SRAM) and a method of operating the same by way of which memorycells that are susceptible to later-life failure caused by biastemperature instability (BTI) can be identified.

Embodiments of this invention provide such a memory and method that canidentify memory cells vulnerable to BTI of either type, namely positivebias temperature instability (PBTI) or negative bias temperatureinstability (NBTI).

Embodiments of this invention provide such a memory and method that canmore directly screen for BTI-susceptible memory cells, rather than byway of an approximation or proxy.

Embodiments of this invention provide such a memory and method that arecapable of accurately and efficiently identifying such susceptiblememory cells so as to minimize unnecessary yield loss.

Embodiments of this invention provide such a memory and method that canbe readily implemented into modern manufacturing technology withoutrequiring a precision photolithography operation.

Embodiments of this invention provide such a memory and method that canincorporate threshold voltage temperature dependence into the screen,avoiding the need to test the memories at temperature.

Embodiments of this invention provide such a memory and method that issuitable for use in connection with high-performance CMOS manufacturingtechnologies such as high-k gate dielectric materials and metal gateelectrodes.

Other objects and advantages of embodiments of this invention will beapparent to those of ordinary skill in the art having reference to thefollowing specification together with its drawings.

Embodiments of this invention may be implemented in connection with asolid-state static random access memory (SRAM) constructed according tocomplementary metal-oxide-semiconductor (CMOS) technology. The SRAMcells are constructed as cross-coupled CMOS inverters, in which theinverter load and driver transistors have their source/drain pathsconnected in series between an array power supply voltage and an arrayground voltage. According to embodiments of this invention, either orboth of the array power supply and array ground voltages applied to thetransistors of each inverter is separately controllable relative to thatapplied to corresponding transistors in the other inverter of the memorycell. Each memory cell is functionally tested by first writing a datastate into the memory cell, followed by reducing the bias to atransistor in one of the inverters from that applied to thecorresponding transistor in the other inverter. In a CMOS arrangement,the reduced bias may be either an increased ground voltage applied tothe source of one of the n-channel driver transistors, or a reducedpower supply voltage applied to the source of one of the p-channel loadtransistors, or both. The cell is then accessed under that asymmetricbias condition. The reduced bias mimics the effect of weakened drivecurrent of a transistor resulting from BTI, allowing evaluation of thevulnerability of the cell to BTI degradation over operating life.

In some embodiments, the memory cells are physically arranged so thatadjacent rows or columns can share a bias voltage line, thus minimizingthe chip area required for separate bias voltages for the inverters ofeach cell. Circuitry outside of the memory array is provided to controlthe application and equalization of the separate bias voltages in testand normal operating conditions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an electrical diagram, in schematic form, of a conventionalstatic random access memory (SRAM) cell.

FIG. 2 is an electrical diagram, in block form, of an integrated circuitincluding one or more memory resources suitable for testing according toembodiments of this invention.

FIG. 3 is an electrical diagram, in block form, of a memory in theintegrated circuit of FIG. 2 suitable for testing according toembodiments of this invention.

FIG. 4 a is an electrical diagram, in schematic form, of an SRAM cellsuitable for testing according to an embodiment of this invention.

FIG. 4 b is an electrical diagram, in schematic form, of bias voltageselect circuitry used in connection with the SRAM cell of FIG. 4 aaccording to that embodiment of the invention.

FIG. 4 c is a plan view of a portion of the memory of FIG. 3 includingthe SRAM cell of FIG. 4 a according to that embodiment of the invention.

FIG. 5 a is an electrical diagram, in schematic form, of an SRAM cellsuitable for testing according to another embodiment of this invention.

FIG. 5 b is an electrical diagram, in schematic form, of bias voltageselect circuitry used in connection with the SRAM cell of FIG. 5 aaccording to that embodiment of the invention.

FIG. 5 c is a plan view of a portion of the memory of FIG. 3 includingthe SRAM cell of FIG. 5 a according to that embodiment of the invention.

FIG. 6 is an electrical diagram, in schematic form, of an SRAM cellsuitable for testing according to another embodiment of this invention.

FIG. 7 is a flow diagram of a method of testing the memory of FIG. 3according to embodiments of this invention.

FIGS. 8 a through 8 d are flow diagrams of screens within the testmethod of FIG. 6, according to an embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in connection with certain embodiments,namely as implemented into a method of testing static random accessmemories, because it is contemplated that this invention will beespecially beneficial when used in such an application. However, it isalso contemplated that embodiments of this invention will also bebeneficial if applied to memories of other types, including read-onlymemories and electrically programmable read-only memories, among others.Furthermore, it is contemplated that embodiments of this invention maybe used to test and screen circuit functions other than memories,including especially digital logic functions. Accordingly, it is to beunderstood that the following description is provided by way of exampleonly, and is not intended to limit the true scope of this invention asclaimed.

FIG. 2 illustrates an example of large-scale integrated circuit 10, inthe form of a so-called “system-on-a-chip” (“SoC”), as now popular inmany electronic systems. Integrated circuit 10 is a single-chipintegrated circuit into which an entire computer architecture isrealized. As such, in this example, integrated circuit 10 includes acentral processing unit of microprocessor 12, which is connected tosystem bus SBUS. Various memory resources, including random accessmemory (RAM) 18 and read-only memory (ROM) 19, reside on system bus SBUSand are thus accessible to microprocessor 12. In many modernimplementations, ROM 19 is realized by way of electrically erasableprogrammable read-only memory (EEPROM), a common type of which isreferred to as “flash” EEPROM. As will be described in further detailbelow, realization of at least part of ROM 19 as flash EEPROM canfacilitate the implementation and operation of embodiments of thisinvention. In any case, ROM 19 typically serves as program memory,storing the program instructions executable by microprocessor 12, whileRAM 18 serves as data memory; in some cases, program instructions mayreside in RAM 18 for recall and execution by microprocessor 12. Cachememory 16 (such as level 1, level 2, and level 3 caches, each typicallyimplemented as SRAM) provides another memory resource, and resideswithin microprocessor 12 itself and therefore does not require busaccess. Other system functions are shown, in a generic sense, inintegrated circuit 10 by way of system control 14 and input/outputinterface 17.

Those skilled in the art having reference to this specification willrecognize that integrated circuit 10 may include additional oralternative functions to those shown in FIG. 2, or may have itsfunctions arranged according to a different architecture from that shownin FIG. 2. The architecture and functionality of integrated circuit 10is thus provided only by way of example, and is not intended to limitthe scope of this invention.

Further detail in connection with the construction of RAM 18 inintegrated circuit 10 is illustrated in FIG. 3. Of course, a similarconstruction may be used to realize other memory resources such as cachememory 16; further in the alternative, RAM 18 may correspond to astand-alone memory integrated circuit (i.e., rather than as an embeddedmemory as shown in FIG. 2). Those skilled in the art having reference tothis specification will comprehend that the memory architecture of RAM18 in FIG. 3 is provided by way of example only.

In this example, RAM 18 includes many memory cells arranged in rows andcolumns within memory array 20. While a single instance of memory array20 is shown in FIG. 3, it is to be understood that RAM 18 may includemultiple memory arrays 20, each corresponding to a memory block withinthe address space of RAM 18. In the example shown in FIG. 3, memoryarray 20 includes m rows and n columns of SRAM cells, with cells in thesame column sharing a pair of bit lines BLT[n-1:0], BLB[n-1:0], and withmemory cells in the same row sharing one of word lines WL[m-1:0]. Bitline precharge circuitry 27 is provided to apply a desired prechargevoltage to the pairs of bit lines BLT[n-1:0], BLB[n-1:0] in advance ofread and write operations. Row decoder 25 receives a row address valueindicating the row of memory array 20 to be accessed, and energizes theone of word lines WL[m-1:0] corresponding to that row address value.Column select circuit 22 receives a column address value, and inresponse selects pairs of bit lines BLT[n-1:0], BLB[n-1:0] associatedwith one or more columns to be placed in communication with read/writecircuits 24. Read/write circuits 24 are constructed in the conventionalmanner, for example to include the typical differential amplifiercoupled to the bit lines for a column as selected by column selectcircuit 22 and a write circuit for selectively pulling toward ground oneof the bit lines in the selected pair. The example of RAM 18 shown inFIG. 3 is constructed to an “interleaved” architecture, in which a givenmemory address selects one of every x (e.g., one of every four) columnsfor read or write access. The data words stored in memory array 20 arethus interleaved with one another, in the sense that the memory addressdecoded (in part) by column select circuit 22 selects one column in eachgroup of columns, along the selected row. Alternatively, memory array 20may be arranged in a non-interleaved fashion, in which each cell in theselected row is coupled to a corresponding read/write circuit in eachcycle. In that architecture, read/write circuits 24 could reside betweenbit lines BL[n-1:0], and column select circuits 22, with the columnselect circuits selecting which read/write circuits 24 (and thus whichcolumns) are in communication with data bus DATA I/O.

As discussed above in connection with the Background of the Invention,modern integrated circuits are now commonly constructed with extremelysmall minimum sized features, for example with metal-oxide-semiconductor(MOS) transistor gates having widths deep in the sub-micron regime.While these small feature sizes provide tremendous cell density and, inmany respects, high device performance, reliability and stability issuesalso result from such scaling. As such, it has become no less importantto properly screen, at the time of manufacture, memory arrays and othercircuit functions in order to identify and repair, or remove from thepopulation, those memory cells and devices that are vulnerable tofailing the desired specifications over operating life. For RAM 18constructed as described above, measures such as static noise margin,writeability, read current, and the like are of particular concern overthe expected operating life.

Furthermore, the extreme thinness required of conventional gatedielectric layers (e.g., silicon dioxide) as transistor feature sizeshave scaled into the deep submicron realm has rendered those materialsunusable in many cases. In response, so-called “high-k” gatedielectrics, such as hafnium oxide (HfO₂), have higher dielectricconstants than silicon dioxide and silicon nitride, permitting thosefilms to be substantially thicker than the corresponding silicon dioxidegate films while remaining suitable for use in high performance MOStransistors. Gate electrodes of metals and metal compounds, such astitanium nitride, tantalum-silicon-nitride, tantalum carbide, and thelike are now also popular in modern MOS technology, especially incombination with high-k gate dielectrics. These metal gate electrodeseliminate effects such as polysilicon depletion, such effects beingnoticeable at the extremely small feature sizes required of thesetechnologies. As discussed above, it has been observed that modernhigh-k metal gate n-channel MOS transistors are susceptible to bothnegative bias temperature instability (“NBTI”) and positive biastemperature instability (“PBTI”).

Accordingly, it would be useful to screen CMOS SRAM cells to identifyand repair, or discard, those memory cells and memories that aresufficiently marginal in static noise margin, writeability, and readcurrent, among other attributes, that transistor degradation over theexpected operating life would result in the loss of a stored data state,a read failure, or a write failure. It is particularly useful to providesuch a screen in the case of SRAM cells constructed of high-k gatedielectric metal gate n-channel MOS transistors, due to their additionalsusceptibility to PBTI. Regardless of the screen, it is important to themanufacturer that such screens accurately test for the contemplateddegradation mechanisms and effects, without significant overkill andthus undue yield loss.

Conventional SRAM cells and memories are not constructed to readilyscreen for these measures, especially as those measures may be affectedby PBTI at the re-channel driver transistors. As such, conventional testvectors necessarily incorporate certain “proxies” for the effects ofshifting device parameters, such as shifts in threshold voltage ofn-channel driver transistors of SRAM cells. However, in order toproperly screen for such effects as BTI, especially at the extremes ofthe temperature range, it has been observed that conventional testvectors often present an unrealistic bias condition to the cells undertest. Such unrealistic test vectors have been observed to introduceother unintended effects and failure modes for the cells, beyond thoserelated to transistor threshold shift. In addition, the necessary testvector voltages that may be applied by level shifters and otherperipheral circuits in the memory architecture are often limited, byvirtue of the design and capability of those peripheral circuits.

According to embodiments of this invention, CMOS SRAM cells areconstructed to enable a more direct screen for bias temperatureinstability (BTI) in the cell transistors. FIG. 4 a illustrates anexample of the construction of memory cell 30 _(jk) of memory array 20,according to embodiments of this invention. Cell 30 _(jk) includes, inthe conventional manner, one CMOS inverter constructed fromseries-connected p-channel load transistor 33 a and n-channel drivertransistor 34 a, and another CMOS inverter of series-connected p-channelload transistor 33 b and n-channel transistor 34 b. The gates oftransistors 33 a, 34 a in one inverter are connected together and to thecommon drain node of transistors 33 b, 34 b of the opposite inverter atstorage node SNB; similarly, the gates of transistors 33 b, 34 b areconnected together and to the common drain node of transistors 33 a, 34a at storage node SNT. N-channel pass-gate transistors 35 a, 35 b havetheir source/drain paths connected between storage nodes SNT, SNB,respectively, and respective bit lines BLT_(k), BLB_(k) for column k ofarray 20. Word line WL_(j) for row j controls the gates of transistors35 a, 35 b. Alternatively, in embodiments of this invention, cell 30_(jk) may be constructed in a “single-sided” manner, in which case onlya single pass-gate transistor coupling one of the storage nodes SNT, SNBto a single bit line will be provided.

According to embodiments of this invention, the source nodes of drivertransistor 34 a and driver transistor 34 b are connected to separatearray ground voltage nodes V_(SSAa), V_(SSAb), respectively. The sourcenodes of load transistors 33 a, 33 b are connected in common to arraypower supply voltage node V_(DDA), and the body nodes of n-channeldriver transistors 34 a, 34 b, and n-channel pass transistors 35 a, 35b, are connected together and biased by a voltage V_(gnd). As will bedescribed below, these separate ground voltage nodes V_(SSAa), V_(SSAb)enable transistors 34 a, 34 b to be placed under different biases fromone another. It is contemplated that the application of asymmetrictransistor biases by way of different array ground voltage nodesV_(SSAa), V_(SSAb) during functional test can mimic threshold voltageshifts at a corresponding one of driver transistors 34 a, 34 b, and alsoto some extent at a corresponding one of pass transistors 35 a, 35 b.More specifically, the application of a voltage above array groundvoltage V_(gnd) at one of array ground voltage nodes V_(SSAa), V_(SSAb)will reduce the bias (i.e., the gate-to-source voltage ordrain-to-source voltage, or both) applied at its corresponding drivertransistor 34 a, 34 b, and perhaps also the bias at its correspondingpass transistor 35 a, 35 b, reducing the drive strength of thattransistor or transistors relative to transistors continuing to receivethe full bias of a nominal ground voltage. The reduced drive strengthcan thus mimic the effect of a higher threshold voltage at the affectedtransistors, and thus the effect of PBTI degradation of those devices asmay occur over operating life.

According to this embodiment of the invention, circuitry is providedoutside of memory array 20 to control the application of voltages toarray ground voltage nodes V_(SSAa), V_(SSAb). FIG. 4 b illustrates anexample of ground voltage select circuitry 40 suitable for use inconnection with this embodiment of the invention. As evident from FIG. 4b, ground voltage select circuitry 40 resides outside of memory array20, and applies a selected voltage to each of array ground voltage nodesV_(SSAa), V_(SSAb), in response to control signals SCRN_a, SCRN_b, andEQ generated by control circuitry (not shown) elsewhere in RAM 18 orintegrated circuit 10 (e.g., system control 14). It is contemplated thatthose skilled in the art having reference to this specification will bereadily able to generate the appropriate levels of control signalsSCRN_a, SCRN_b, EQ for carrying out the normal operation and screenfunctions described below in this specification.

As shown in FIG. 4 b, ground voltage select circuitry 40 receives twopower supply voltages as inputs, namely normal ground voltage V_(gnd)(which, in this example, is the bias voltage applied to body nodes ofn-channel transistors 34, 35 in cell 30 _(jk) as discussed above) andscreen ground voltage V_(SS) _(—) _(SCRN). It is contemplated thatvoltage regulator or level shift circuitry elsewhere in RAM 18 orintegrated circuit 10 can generate screen ground voltage V_(SS) _(—)_(SCRN) at a different (i.e., higher) voltage than normal ground voltageV_(gnd). On its array side, ground voltage select circuitry 40 isconnected to array ground voltage nodes V_(SSAa), V_(SSAb); as such, thefunction of ground voltage select circuitry 40 is to apply theappropriate ones of normal ground voltage V_(gnd) and screen groundvoltage V_(SSA) _(—) _(SCRN) to array ground voltage nodes V_(SSAa),V_(SSAb) for the particular operating mode or screen during test.

In this embodiment of the invention, ground voltage select circuitry 40includes n-channel MOS pass transistor 42 a having its source/drain pathconnected between screen ground voltage V_(SSA) _(—) _(SCRN) and arrayground voltage node V_(SSAa), and re-channel pass transistor 44 a havingits source/drain path connected between normal ground voltage V_(gnd)and array ground voltage node V_(SSAa). The gate of transistor 42 areceives control signal SCRN_a, and the gate of transistor 44 a receivesthe logical complement of control signal SCRN_a (via inverter 41 a).Similarly, n-channel pass transistor 42 b has its source/drain pathconnected between screen ground voltage V_(SSA) _(—) _(SCRN) and arrayground voltage node V_(SSAb), and n-channel pass transistor 44 b has itssource/drain path connected between normal ground voltage V_(gnd) andarray ground voltage node V_(SSAb). The gate of transistor 42 b receivescontrol signal SCRN_b, and the gate of transistor 44 b receives thelogical complement of control signal SCRN_b (via inverter 41 b).Equalization transistor 43 is an n-channel transistor with itssource/drain path connected between array ground voltage nodes V_(SSAa)and V_(SSAb), and its gate receiving equalization signal EQ.Equalization signal EQ may be generated by external control circuitry asnoted above, or alternatively may be produced by logic circuitryaccording to control signals SCRN_a, SCRN_b (e.g., the logical AND ofthe outputs of inverters 41 a, 41 b).

In normal operation, control signals SCRN_a, SCRN_b will both beinactive at a low logic level. Transistors 42 a, 42 b will both be off,and transistors 44 a, 44 b will both be on, in ground voltage selectcircuitry 40. This will apply normal ground voltage V_(gnd) to both ofarray ground voltage nodes V_(SSAa), V_(SSBb). In this normal mode, itis contemplated that equalization control signal EQ will be active at ahigh logic level, turning on equalization transistor 43 to ensuresymmetric application of normal ground voltage V_(gnd) to array groundvoltage nodes V_(SSAa), V_(SSBb). In operation during test, for exampleto carry out an asymmetric bias screen in the manner described infurther detail below, one or the other of control signals SCRN_a, SCRN_bwill be asserted active, while the other remains inactive. For example,in response to control signal SCRN_a being driven active and controlsignal SCRN_b remaining inactive, transistor 42 a will be turned on andtransistor 44 a will be off, which connects screen ground voltageV_(SSA) _(—) _(SCRN) to array ground voltage node V_(SSAa); transistors42 b, 44 b will be off and on, respectively, connecting normal groundvoltage V_(gnd) to array ground voltage node V_(SSAb). Equalizationtransistor 43 will be turned off in this situation (equalization signalEQ inactive), to allow array ground voltage nodes V_(SSAa), V_(SSAb) toreach different voltages from one another, and thus result in anasymmetric bias at cells 30 in memory array 20. Conversely, with controlsignal SCRN_b active and control signal SCRN_a inactive, array groundvoltage node V_(SSAb) will receive screen ground voltage V_(SSA) _(—)_(SCRN) and array ground voltage node V_(SSAa) will receive normalground voltage V_(gnd).

It is contemplated that multiple cells 30 within memory array 20 willreceive array ground voltage nodes V_(SSAa), V_(SSAb) according to thisembodiment of the invention. For example, it is contemplated that everycell 30 within memory array 20 will receive array ground voltage nodesV_(SSAa), V_(SSAb) in similar manner as described above for the case ofcell 30 _(jk) of FIG. 4 a. Alternatively, it is contemplated thatmultiple instances of ground voltage select circuitry 40, and thusmultiple instances of paired array ground voltage nodes V_(SSAa),V_(SSAb), may be provided in connection with memory array 30. In anyevent, it is contemplated that cells 30 in the same column k as cell 30_(jk) (or in the same row j, depending on the orientation of memoryarray 20 relative to the run of conductors corresponding to array groundvoltage nodes V_(SSAa), V_(SSAb)) will share array ground voltage nodesV_(SSAa), V_(SSAb).

It is contemplated that conductors corresponding to array ground voltagenodes V_(SSAa), V_(SSAb) can be readily routed through memory array 20,providing the capability for the PBTI screens described in furtherdetail below, without significantly increasing the chip area requiredfor these separate conductors. FIG. 4 c illustrates an example of thelayout of cell 30 _(jk) of FIG. 4 a, and portions of its immediatelyadjacent cells within the same row j (i.e., in columns k−1 and k+1) asconstructed at the surface of a silicon substrate, fabricated accordingto CMOS technology and according to an embodiment of this invention, andat a stage in the manufacture prior to the formation of overlying metallayers. As will become apparent, chip area efficiency is accomplished byrouting array ground voltage node conductors so that cells in adjacentcolumns can share a common array ground voltage node conductor, as willnow be described.

In the plan view of FIG. 4 c, active regions 54 are locations of thesurface of an n-well or a p-well, as the case may be, at whichdielectric isolation structures 53 are not present. As known in the art,isolation dielectric structures 53 are relatively thick structures ofsilicon dioxide or another dielectric material, intended to isolationtransistor source and drain regions in separate transistors from oneanother. Isolation dielectric structures 53 are typically formed by wayof shallow trench isolation (STI) structures in modern high-densityintegrated circuits, or alternatively by the well-known local oxidationof silicon (LOCOS) process. FIG. 4 c illustrates the boundaries ofp-wells 52 and n-wells 55, within which active regions 54 are defined.These well regions will receive the appropriate body node bias via metalconductors (not shown), in the conventional manner.

As well known in the art, transistors are formed at locations of activeregions 54 that underlie gate elements 56, separated therefrom by gatedielectric material (not visible in FIG. 4 c). Various materials may beused for gate element 56 and for this gate dielectric, as describedabove. Commonly used materials include polycrystalline silicon for gateelement 56, and silicon dioxide or silicon nitride (or a combination ofthe two) for the gate dielectric. Those conventional materials aresuitable for use with embodiments of this invention. High-k dielectricmaterials such as hafnium oxide (HfO₂) are becoming favored for use inhigh-performance transistors, in combination with metals or conductivemetal compounds for gate elements 56, examples of which include titaniumnitride, tantalum silicon nitride, and tantalum carbide. Other examplesof these high-k gate dielectric materials and metal gate materials areknown in the art. It is contemplated that embodiments of this inventionare especially well-suited for use in connection with such moderntransistor materials, particularly considering that these embodiments ofthe invention can readily screen for device instabilities such as PBTI,to which transistors using those modern materials have been observed tobe susceptible.

FIG. 4 c illustrates the locations of contact openings 58 that extendthrough overlying insulator material (not shown) to active regions 54 orto gate elements 56, at the case may be. Metal conductors (e.g., thoseshown schematically in FIG. 4 c for storage nodes SNT, SNB) will bepatterned to form conductors that overlie the structure, making contactto active regions 54 or gate elements 56 (or both) via respectivecontact openings 58.

FIG. 4 c illustrates the outline of the various transistors 33, 34, 35within cell 30 _(jk), corresponding to the electrical schematic of FIG.4 a. It is contemplated that those skilled in the art will be able tofollow the schematic of FIG. 4 a within the layout of FIG. 4 c. Theorientation of the plan view of FIG. 4 a is such that cell 30 _(j,k) inrow m and column k is highlighted. Above and below SRAM cell 30 _(m,k)are cells 30 _(m,k−1), 30 _(m,k+1), which also reside in the same row mbut in columns k and k+1, respectively. SRAM cell 30 _(m,k) is along theright-hand side of cell 30 _(m,k), residing in the same column k but inneighboring row m+1.

As mentioned above, the metal conductor schematically shown as storagenode SNB connects active region 54 at the drain of transistor 34 b andone side of pass transistor 35 b to active region 54 at the drain oftransistor 33 b and to gate element 56 serving as the gate oftransistors 33 a, 34 a (via a shared contact opening 58). Similarly, themetal conductor schematically shown as storage node SNT connects activeregion 54 between transistors 34 a, 35 a to active region 54 at thedrain of transistor 33 a, and (via shared contact opening 58) to gateelement 56 serving as the gates of transistors 33 b, 34 b. The routingof array power supply node conductor V_(DDA) is illustrated in FIG. 4 c,including those locations at which it makes contact (via contactopenings 58) to the source nodes of transistors 34 a, 34 b. Bit linesBLT_(k), BLB_(k), and word line WL_(j) are connected, via metalconductors (not shown) and contact openings 58 to the appropriateelements within cell 30 _(jk) as shown in FIG. 4 c, according to theelectrical schematic of FIG. 4 a.

According to this embodiment of the invention, metal conductors forarray ground voltage nodes V_(SSAa), V_(SSAb) run parallel to theconductor for array power supply voltage node V_(DDA), as shownschematically in FIG. 4 c. The metal conductor for array ground voltagenode V_(SSAa) connects, via contact openings 58, to the source node oftransistor 34 a in cell 30 _(jk) and also to the source node oftransistor 34 a of cell 30 _(j,k+1) in adjacent column k+1. Similarly,the metal conductor for array ground voltage node V_(SSAb) connects, viacontact openings 58, to the source node of transistor 34 b in cell 30_(jk) and also to the source node of transistor 34 b of cell 30 _(j,k−1)in adjacent column k−1. As such, it is contemplated that separate arrayground voltage nodes V_(SSAa), V_(SSAb) within memory array 20 can beimplemented with little if any impact on the overall cell chip area.

Referring now to FIGS. 5 a through 5 c, the construction of SRAM cell30′_(jk) and associated support circuitry within RAM 18 according toanother embodiment of this invention will now be described in detail.Cell 30′_(jk) is constructed in similar manner as cell 30 _(jk) of FIG.4 a, with a pair of cross-coupled CMOS inverters, one of whichconsisting of p-channel load transistor 33 a and n-channel drivertransistor 34 a, and the other as p-channel load transistor 33 b andn-channel transistor 34 b. The cross-coupling of these two inverters iseffected by the gates of transistors 33 a, 34 a both connected to thecommon drain node of transistors 33 b, 34 b together at storage nodeSNB, and by the gates of transistors 33 b, 34 b connected together tothe common drain node of transistors 33 a, 34 a at storage node SNT.Pass-gate transistors 35 a, 35 b respectively selectively couple storagenodes SNT, SNB to bit lines BLT_(k), BLB_(k), respectively, of column kin response to the activation of word line WL_(j) for row j.

According to this embodiment of this invention, the source nodes of loadtransistor 33 a and load transistor 33 b are connected to separate arraypower supply voltage nodes V_(DDAa), V_(DDAb), respectively. The sourcenodes of driver transistors 34 a, 34 b are connected in common to arrayground voltage node V_(SSA), and the body nodes of p-channel loadtransistors 33 a, 33 b are connected in common to array power supplyvoltage V_(DDA). The separate array power supply voltage nodes V_(DDAa),V_(DDAb) enable transistors 33 a, 33 b to receive different biases fromone another. More specifically, the application of a voltage below arraypower supply voltage V_(DDA) at one of array power supply voltage nodesV_(DDAa), V_(DDAb) will asymmetrically reduce the bias (i.e., thegate-to-source voltage, the drain-to-source voltage, or both) of itscorresponding load transistor 33 a, 33 b, reducing its drive in a mannersimilar to the effect of NBTI degradation. This asymmetric bias is thususeful in performing a time-zero screen for cells 30′ that arevulnerable to failure due to NBTI degradation.

Similarly as shown in FIG. 4 b described above, FIG. 5 b illustrates anexample of power supply voltage select circuitry 45 suitable for use inconnection with cell 30′_(jk) according to this embodiment of theinvention. As before, power voltage select circuitry 45 resides outsideof memory array 20, and selectively applies voltages to array powersupply voltage nodes V_(DDAa), V_(DDAb), in response to control signalsSCRN_a/, SCRN_b/, and EQ/ generated by control circuitry (not shown)elsewhere in RAM 18 or integrated circuit 10 (e.g., system control 14).The “/” signal designator for these signal lines indicates that thesecontrol signals are active at a low logic level. It is contemplated thatthose skilled in the art having reference to this specification will bereadily able to generate the appropriate levels of control signalsSCRN_a/, SCRN_b/, EQ/ for carrying out the normal operation and screenfunctions described below in this specification.

Power supply voltage select circuitry 45 is constructed similarly asground voltage select circuitry 40 described above in connection withFIG. 4 b, in essentially a complementary fashion. As such, power supplyvoltage select circuitry 45 includes p-channel MOS pass transistor 46 awith its source/drain path connected between screen power supply voltageV_(DDA) _(—) _(SCRN) and array power supply voltage node V_(DDAa), andp-channel pass transistor 48 a having its source/drain path connectedbetween normal power voltage V_(DDA) and array power voltage nodeV_(DDAa). The gate of transistor 46 a receives control signal SCRN_a/,and the gate of transistor 48 a receives the logical complement ofcontrol signal SCRN_a/ via inverter 47 a. Similarly, p-channel passtransistor 46 b has its source/drain path connected between screen powersupply voltage V_(DDA) _(—) _(SCRN) and array power supply voltage nodeV_(DDAb), and p-channel pass transistor 48 b has its source/drain pathconnected between normal power supply voltage V_(DDA) and array powersupply voltage node V_(DDAb). The gate of transistor 46 b receivescontrol signal SCRN_b/, and the gate of transistor 48 b receives thelogical complement of control signal SCRN_b/ via inverter 47 b.Equalization transistor 49 is a p-channel transistor with itssource/drain path connected between array power supply voltage nodesV_(DDAa) and V_(DDAb), and its gate receiving equalization signal EQ/.Equalization signal EQ/ may be generated by external control circuitryas noted above, or alternatively may be produced by logic circuitry asthe logical NAND of control signals SCRN_a/, SCRN_b/ or in some otherlogical arrangement.

Power supply voltages V_(DDA) and screen power supply voltage V_(DD)_(—) _(SCRN) may be generated by conventional voltage regulator or levelshift circuitry elsewhere in RAM 18 or in integrated circuit 10. Forpurposes of this embodiment of the invention, screen power supplyvoltage V_(DD) _(—) _(SCRN) will typically be at a lower voltage thannormal power supply voltage V_(DDA).

In normal operation, control signals SCRN_a/, SCRN_b/ will both beinactive (at a high logic level), which turns off both of transistors 46a, 46 b and turns on transistors 48 a, 48 b. Typically, equalizationcontrol signal EQ/ will be at an active low level during normaloperation, turning on transistor 49. In this condition, normal powersupply voltage V_(DDA) is applied to both of array power supply voltagenodes V_(DDAa), V_(DDAb), with equalization transistor 49 ensuringsymmetric application of that voltage. During a time-zero screen test,one or the other of control signals SCRN_a/, SCRN_b/ will be assertedactive (low), while the other remains inactive (high); equalizationcontrol signal EQ/ will be driven inactive (high). Screen power supplyvoltage V_(DDA) _(—) _(SCRN) will be applied to the one of array powersupply nodes V_(DDAa), V_(DDAb) connected to the one of transistor 46 a,46 b turned on by the active one of control signals SCRN_a/, SCRN_b/,applying an asymmetric bias to load transistors 33 a, 33 b in one ormore cells 30′ within memory array 20.

Similarly as described above in connection with FIGS. 4 a through 4 c,conductors corresponding to array power supply voltage nodes V_(DDAa),V_(DDAb) can be readily routed through memory array 20, withoutsignificantly increasing the chip area required for these separateconductors. FIG. 5 c illustrates an example of the layout of cell30′_(jk) of FIG. 5 a, as constructed at the surface of a siliconsubstrate, fabricated according to CMOS technology and according to anembodiment of this invention, and at a stage in the manufacture prior tothe formation of overlying metal layers.

The arrangement of the transistors, active regions 54, gate elements 56,contact openings 58, and isolation dielectric structures 53, of cell30′_(jk) is essentially identical to that described above in connectionwith FIG. 4 c. Cell 30′_(jk) differs, however, in connection with therouting of metal conductors for array power supply voltage nodesV_(DDAa), V_(DDAb). In this embodiment of the invention, the metalconductor for array power supply voltage node V_(DDAa) runs parallel tothe conductor for array ground voltage node V_(SSA), and connects to thesource node of transistor 33 a in cell 30′_(jk) via a contact opening58. Similarly, the metal conductor for array power supply voltage nodeV_(DDAb) connects to the source node of transistor 33 b in cell 30′_(jk)via a corresponding contact opening 58.

As evident from FIG. 5 c, the metal conductors for array power supplyvoltage nodes V_(DDAa), V_(DDAb) are not shared between adjacent columnsin this implementation. However, it is contemplated that the chip areaeffect of this routing in the layout of cell 30′_(jk) will be minimal,particularly if the metal conductors for array power supply voltagenodes V_(DDAa), V_(DDAb) can run directly over their respective contactopenings 58 to transistors 33 a, 33 b as shown. Alternatively, it iscontemplated that cell 30′_(jk) can be laid out in such a manner as toenable the sharing of these metal conductors between adjacent columns.

FIG. 6 illustrates cell 30″_(jk) constructed according to anotherembodiment of this invention. Cell 30″_(jk) is constructed in similarmanner as cells 30 _(jk) and 30′_(jk) of FIGS. 4 a and 5 a,respectively. In particular, cell 30″_(jk) is constructed as a “6-T”SRAM cell in which the cross-coupled CMOS inverters may be biasedseparately from one another both at the source node of the p-channelload transistor and at the source node of the n-channel drivertransistor. More specifically, according to this embodiment of thisinvention, the source nodes of p-channel load transistors 33 a, 33 b areconnected to separate array power supply voltage nodes V_(DDAa),V_(DDAb), respectively; in addition, the source nodes of n-channeldriver transistors 34 a, 34 b are connected to separate array groundvoltage nodes V_(SSAa), V_(SSAb), respectively. The body nodes of loadtransistors 33 a, 33 b are connected in common to array power supplyvoltage V_(DD) and the body nodes of driver transistors 34 a, 34 b areconnected in common to array ground voltage V_(gnd). As such, accordingto this embodiment, load transistors 33 a, 33 b can be asymmetricallybiased by way of a voltage below array power supply voltage V_(DD)applied to one of array power supply voltage nodes V_(DDAa), V_(DDAb),and driver transistors 34 a, 34 b can be asymmetrically biased by way ofa voltage above array ground voltage V_(gnd) applied to one of arrayground voltages nodes V_(SSAa), V_(SSAb). This asymmetric bias is thususeful in performing a time-zero screen for cells 30′ that arevulnerable to failure due either or both of PBTI and NBTI degradation.

It is contemplated that memory array 20 constructed with cells 30″ asdescribed above in connection with FIG. 6 will include both power supplyvoltage select circuitry 45 and also ground voltage select circuitry 40.In this regard, it is contemplated that the respective control signalsSCRN_a, SCRN_b, SCRN_a/, SCRN_b/, EQ, EQ/ can be combined and generatedin an efficient manner by the appropriate control circuitry. It isfurther contemplated that the layout of FIGS. 4 c and 5 c can be readilymodified by those skilled in the art having reference to thisspecification so as to incorporate both pairs of power supply and groundconductors V_(DDAa), V_(DDAb), V_(SSAa), V_(SSAb) as appropriate.

It is of course contemplated that the particular schematic circuitarrangement, physical layout, and construction of memory array 20 andits constituent SRAM cells 30, 30′, 30″ may vary significantly from thatshown in FIGS. 4 a through 4 c, 5 a through 5 c, and 6 and describedherein, such variations and alternatives being apparent to those skilledin the art having reference to this specification, yet remain within thescope of this invention. It is therefore to be understood that thisdescription of the architecture, layout, and construction of memoryarray 20 and SRAM cells 30, 30′, 30″ is provided by way of example only.

Referring now to FIG. 7, a method of testing and screening SRAM cells 30within RAM 18 of integrated circuit 10, or as a stand-alone memoryintegrated circuit, as the case may be, according to embodiments of thisinvention will now be described. It is contemplated that the testprocess of embodiments of this invention, which involves the applicationof asymmetric bias voltages to inverter transistors within cells 30under test within memory array 20 as will be described below, is wellsuited for wafer-level testing (i.e., “multiprobe” functional testing),prior to packaging of integrated circuit 10, because of the ability toreplace marginal cells by way of conventional redundancy techniques atthat stage of manufacture. Of course, the test process of FIG. 7 mayadditionally or instead be performed after packaging, as desired.

It is contemplated that the method of FIG. 7 will typically be performedby way of automated test equipment, for example automated test equipmentas used in functionally testing integrated circuits 10. The method ofFIG. 7 will be described in connection with the testing of a populationof memory cells, for example the testing of array 20 of RAM 18 of FIG.3. It is contemplated that the particular test sequence mayalternatively be applied fully to each memory cell in sequence (i.e.,the entire test sequence performed for each cell 30 in turn). Further inthe alternative, the test sequence may be applied to cells 30 in a row,column, or sub-array of array 20, or to some other population smallerthan the entire array 20. As such, while the method described below inconnection with FIG. 7 will refer to a population of cells 30 undertest, it is to be understood that the number of cells 30 in thatpopulation can number from one to the entire array 20. It iscontemplated that those skilled in the art having reference to thisspecification will be readily able to apply the test sequence of FIG. 7to the appropriate number of memory cells 30 for specific memoryarchitectures.

The manufacturing test flow shown in FIG. 7 according to embodiments ofthe invention begins with process 60 in which conventional parametrictests of both the DC and operating type (e.g., continuity, leakage,standby and active power dissipation, etc.) are performed upon RAM 18under test. As described above and as will be described in furtherdetail below, the screens according to embodiments of this invention areintended to identify those SRAM cells 30 that are vulnerable to failureover operating life. As such, functional tests are performed by theautomated test equipment, in process 62, to evaluate the ability of RAM18 to be written and read with both data states under such operatingconditions and timing constraints required by specifications. Accordingto these embodiments of the invention, parametric test process 60 andfunctional test process 62 are contemplated to be performed using“normal” bias voltages applied symmetrically to the inverters of SRAMcells 30. These “normal” bias voltages applied to cells 30 refer toarray power supply voltage levels and array ground voltage levelsconsistent with normal operating specifications and tolerances for RAM18. For example, if the performance of RAM 18 is specified over a rangeof array power supply voltage (V_(DDA), for example) of 1.20 volts±5%relative to the array ground voltage (V_(SSA), for example), a “normal”bias applied to the series-connected load and driver transistors of theinverters of SRAM cell 30 _(jk) will be 1.20 volts±5%, adjusted by anyguardbanding in the test process associated with the specifiedtemperature range, noise margin, and the like. The “symmetric” biasescontemplated to be applied in processes 60, 62 refers to the applicationof essentially identical voltages to both load transistors, and to bothdriver transistors, within each SRAM cell 30 under test. Processes 60and 62 thus remove, from the population of SRAM cells 30 or ofintegrated circuits 10 in the aggregate, those devices that do not meetthe time-zero specifications desired of those functions. This ensuresthat the screen processes performed according to embodiments of thisinvention are applied only to devices that are known to be functionallyand parametrically acceptable at normal and symmetric biases.Alternatively, the voltages applied in processes 60, 62 may stray fromthe “normal” range as desired by the test designer, so long as a known“0” data state is written into cells 30 under test.

According to this embodiment of the invention, asymmetric bias screenprocess 64 is then performed by the automated test equipment on one ormore SRAM cells 30 in RAM 18, to determine whether any of those cells 30may be vulnerable to shifts in transistor characteristics due to PBTI-and NBTI-caused threshold voltage shifts insofar as such shifts affectthe read performance, write performance, read stability, and retentionstability of cells 30 under test. According to embodiments of thisinvention, one or more particular test sequences are included withinasymmetric bias screen process 64, each such sequence applying reducedbias to transistors in one of the inverters within each cell 30 undertest in order to determine whether that cell is vulnerable to thresholdvoltage degradation due to PBTI or NBTI, as the case may be.

FIG. 8 a illustrates a method for performing one of these testsequences, namely read stability test sequence 64 a as applied to one ormore SRAM cells 30. For purposes of the following description, includingthe description in connection with FIGS. 8 b and 8 c, reference will bemade to SRAM cells 30 under test, with the understanding that such cells30 refer to SRAM cells constructed as any of cells 30 _(jk), 30′_(jk),30″_(jk) described above or variations thereof. As shown in FIG. 8 a,test sequence 64 a in this example begins with the writing of a knowndata state (e.g., “0”) into each of the SRAM cells 30 under test, inprocess 70. Process 70 is typically performed under normal biasconditions; if desired, a read of those SRAM cells 30 may be performedto verify the written data state into these cells.

As described above, the primary effect of PBTI is a positive shift inthe threshold voltage of n-channel transistors, particularly those atwhich a positive gate voltage (relative to the transistor channelregion) has been present for some duration. Conversely, the primaryeffect of NBTI is a positive shift in the threshold voltage of p-channeltransistors receiving a negative gate voltage for some duration. In eachcase, the increased threshold voltage weakens the drive of thetransistor, slowing its switching and also reducing its source/draincurrent in the “on” state. Also as discussed above, read stabilityfailures occur when the threshold voltage has shifted for either or bothof the “on” state load transistor 33 a, 33 b or the “on” state drivertransistor 34 b, 34 a for the current data state. Following process 70,load transistor 33 b and driver transistor 34 a are the “on” statetransistors.

According to this embodiment of the invention, the effect of BTIthreshold shift is mimicked by the application of a reduced bias toeither or both of the “on” state transistors, namely load transistor 33b and driver transistor 34 a, for each SRAM cell 30 currently undertest. The particular transistor receiving the reduced bias will ofcourse depend on the construction of SRAM cell 30, according to theexamples described above relative to FIGS. 4 a, 5 a and 6. For theexample of cell 30 _(jk) of FIG. 4 a, a higher voltage will be appliedat array ground voltage node V_(SSAa) than at array ground voltage nodeV_(SSAb) in process 72. It is contemplated that the voltage differentialbetween array ground voltage nodes V_(SSAa) and V_(SSAb) will be on theorder of 10% of the nominal bias voltage for the inverters of cell 30.For example, if array power supply voltage V_(DDA) is nominally at 1.2volts above normal ground voltage V_(gnd), array ground voltage nodeV_(SSAa) may receive a voltage that is about 100 mV above array groundvoltage V_(gnd). To further tighten the screen, array power supplyvoltage V_(DDA) may be set at its minimum specification level (e.g., 1.0volts) in process 72.

Of course, in the cases in which the alternative constructions of cells30′_(jk) and 30″_(jk) are implemented in memory array 20, the asymmetricbias applied in process 72 may instead or additionally apply a lowervoltage to array power supply voltage node V_(DDAb) than at array powersupply voltage node V_(DDAa), reducing the bias at load transistor 33 bfor the case of this “0” data state.

Once the asymmetric transistor bias is applied in process 72, readdisturb process 74 is then performed by the automated test equipment,upon the cells under test. The particular nature of the disturb cycle orcycles applied in process 74 may vary, depending on the expected readstability vulnerability mechanism. Typically, it is contemplated thatprocess 74 will include an access (read or write) to a cell 30 that isin the same row as each cell 30 under test, to ensure that passtransistor 35 a is turned on for the cells 30 under test (i.e., cell 30is “half-selected”). As discussed above, a typical stability failure iscaused by the inability of the “on” state driver transistor 34 a tomaintain a low level at storage node SNT, for the “0” data state case,if pass transistor 35 a is turned on so as to couple storage node SNT tothe precharged bit line BLT_(k). Other disturb operations mayadditionally or instead be included within process 72, including a readof each cell under test itself, a write of an opposite data state to acell 30 in the same column as each cell under test, or the like. Thoseskilled in the art having reference to this specification will readilyidentify those disturb conditions suitable for inclusion within process74.

Following read disturb process 74, normal and symmetric bias is thenapplied to all transistors of cells 30 under test, in process 76. Cells30 under test are then read in process 77, to determine whether the readdisturb of process 74 under the reduced bias of process 72 caused lossof the stored data state (i.e., the “0” data state written in process70). The addresses of failing cells may be stored by the automated testequipment for use in redundant repair, or alternatively a first failedcell may trigger a “fail” condition for RAM 18 as a whole.

In decision 78, the automated test equipment determines whether bothdata states have been tested. If not (decision 78 is “no”), a “1” datastate is then written into cells 30 under test in process 79,considering that “normal” bias remains applied as a result of process76. Processes 72, 74, 76, 77 are then repeated for this opposite datastate in the manner described above, except that the reduced bias willbe applied to driver transistor 34 b, load transistor 33 a, or both,depending on the particular cell construction. For the example of cell30 _(jk) of FIG. 4 a, of course, a higher voltage will be applied atarray ground voltage node V_(SSAb) than at array ground voltage nodeV_(SSAa) in this instance of process 72 for the “1” data state.Following this pass of the test sequence for the “1” data state(decision 78 is “yes”), read stability test sequence 64 a is complete.

FIG. 8 b illustrates the method for performing read current testsequence 64 b as applied to one or more SRAM cells 30, also as part ofscreen process 64. As discussed above, the effects of PBTI degradationcan weaken the read current sourced at the one of driver transistors 34a, 34 b holding a low data state at its storage node; conversely, NBTIdegradation can cause the opposite load transistor 33 a, 33 b to apply areduced gate voltage at the “on” state driver in the opposing inverter,similarly weakening the read current. Read current test sequence 64 bscreens cells 30 under test for these potential weaknesses in readcurrent.

Test sequence 64 b in this example begins with the writing of a knowndata state (e.g., “0”) into each of the SRAM cells 30 under test, inprocess 80, under normal bias. As before, a read of those SRAM cells 30may be performed to verify the written data state into these cells. Forthis “0” data state, load transistor 33 b and driver transistor 34 a areagain the “on” state transistors. In process 82 according to thisembodiment of the invention, a reduced bias is applied to either or bothof the “on” state transistors, namely load transistor 33 b and drivertransistor 34 a, for each SRAM cell 30 under test. This asymmetric biasapplied in process 82 is essentially the same as described above inconnection with process 72 of FIG. 8 a, and as such the particulartransistor receiving the reduced bias depends on the construction ofSRAM cell 30, according to the examples described above relative toFIGS. 4 a, 5 a and 6.

Under this reduced transistor bias as applied in process 82, each ofcells 30 under test is read by the automated test equipment in process84. The reduction in transistor bias serves to mimic the effect ofthreshold voltage degradation due to BTI, as that reduced bias weakensthe transistor drive. The read of process 84 thus tests the read currentunder the reduced bias (e.g., a raised voltage at the source of drivertransistor 34 a applied by array ground voltage node V_(SSAa)), thusdetermining whether that read current is sufficient to convey thecorrect “0” data state.

Following the read of process 84, the automated test equipmentdetermines whether both data states have been tested in decision 85. Ifnot (decision 85 is “no”), normal symmetric bias is then applied to alltransistors of cells 30 under test in process 86, and the opposite “1”data state is written into cells 30 under test in process 87. Processes82, 84, 85 are then repeated for this opposite data state in the mannerdescribed above, except that the reduced bias will be applied to drivertransistor 34 b, load transistor 33 a, or both, depending on theparticular cell construction. Following this pass of the test sequencefor the “1” data state (decision 85 is “yes”), read test sequence 64 bis complete. The addresses of any failing cells 30 are stored for use inredundant repair, or alternatively RAM 18 is considered as “failed” iffailing cells were detected.

FIG. 8 c illustrates a method for performing writeability test sequence64 c as may be included within screen process 64, according toembodiments of this invention. This test sequence 64 c in this examplebegins with the writing of the “0” data state into each of the SRAMcells 30 under test, in process 90. As before, for this “0” data state,load transistor 33 b and driver transistor 34 a are again the “on” statetransistors.

As discussed above, write failures are commonly due to weakness in apass transistor 35 a, 35 b to allow the low-side bit line to overcomethe drive of its associated load transistor in pulling its storage nodefrom high to low, and in weakness in the associated driver transistor 34a, 34 b to responding to feedback to change the cell state, both due toPBTI in the cell construction described above. NBTI degradation at aload transistor 33 a, 33 b can also cause a write failure, as weakerdrive current will degrade its ability to pull a storage node from lowto high in response to the write. As such, write screen process 64 cwill reduce the bias voltage to “off” state transistors, namely eitheror both of driver transistor 34 b and load transistor 33 a, which willalso affect pass transistor 35 b at storage node SNB (which is at a highlevel for this “0” state).

In process 92 according to this embodiment of the invention, therefore,BTI threshold shift is mimicked by the application of a reduced bias toeither or both of the “off” state transistors, namely driver transistor34 b and load transistor 33 a, for each SRAM cell 30 currently undertest. This asymmetric bias applied in process 92 is essentially the sameas described above in connection with processes 72, 82, but is appliedto the opposite transistor or transistors from those screen processes 64a, 64 b, and will depend on the construction of SRAM cell 30, accordingto the examples described above relative to FIGS. 4 a, 5 a and 6. Forthe example of cell 30 _(jk) of FIG. 4 a, a higher voltage will beapplied at array ground voltage node V_(SSAb) than at array groundvoltage node V_(SSAa) in process 92; for example, this voltagedifferential can be on the order of 10% of the nominal bias voltage forthe inverters of cell 30 (e.g., 100 mV for nominal array power supplyvoltage V_(DDA) of 1.2 volts above normal ground voltage V_(gnd)).

The automated test equipment then performs a write of the opposite (“1”)data state to cells 30 under test in process 94, under the reducedtransistor bias applied in process 92 that mimics threshold voltagedegradation due to BTI. This write operation of process 94 thus teststhe ability of pass transistor 35 b and driver transistor 34 b, or loadtransistor 33 a (or all those transistors) to successfully change thestate of cell 30 under test at the reduced drive current caused by thereduced bias (e.g., a raised voltage at the source of driver transistor34 b applied by array ground voltage node V_(SSAb)). Following writeprocess 94, normal symmetric bias is then applied to all transistors ofcells 30 under test, in process 96, and cells 30 under test are thenread in process 97 to determine whether the write of process 94 underthe reduced bias of process 92 was successful. The addresses of failingcells may be stored by the automated test equipment for use in redundantrepair, or alternatively a first failed cell may trigger a “fail”condition for RAM 18 as a whole.

In decision 98, the automated test equipment determines whether bothdata states have been tested. If not (decision 98 is “no”), a “1” datastate is then written into cells 30 under test in process 99, andprocesses 92, 94, 96, 97 are then repeated for this opposite data stateas described above. Of course, the asymmetric bias applied in process 92will be reversed for this data state, to evaluate operation should theother transistors be weakened. Following this pass of the write testsequence 64 c for the “1” data state (decision 98 is “yes”), write testsequence 64 c is complete. The addresses of any failing cells 30 arestored for use in redundant repair, or alternatively RAM 18 isconsidered as “failed” if failing cells were detected.

FIG. 8 d illustrates a method for performing retention-disturb testsequence 64 d as may also be included within screen process 64,according to embodiments of this invention. In process 100, theautomated test equipment writes a known data state (e.g., “0”) into eachof the SRAM cells 30 under test, under normal bias, followed by a read(if desired) to verify the data states written.

In process 102, a reduced bias is applied to either or both of the “on”state transistors, namely load transistor 33 b and driver transistor 34a for a “0” data state, in each SRAM cell 30 currently under test. Asdescribed above, for the example of cell 30 _(jk) of FIG. 4 a, a highervoltage is applied at array ground voltage node V_(SSAa) than at arrayground voltage node V_(SSAb) in process 102, for example a voltage onthe order of 10% of the nominal inverter bias voltage (i.e., about 100mV above array ground voltage V_(gnd) at a nominal array power supplyvoltage of 1.2 V). Also in process 102, array power supply voltageV_(DDA) may be set at its minimum specification level (e.g., 1.0 volts),if a more stringent screen is desired. The result of process 102 is tomimic the effect of PBTI degradation at the “on” drive transistor(driver transistor 34 a for a “0” data state), by way of weakening itsdrive by reducing its gate-to-source and drain-to-source voltagesrelative to that of opposing driver transistor 34 b.

Following application of the reduced transistor bias in process 102,retention disturb process 104 is then performed by the automated testequipment on the cells under test. According to this embodiment of theinvention, in which the reduced bias of process 102 consists of a higherground level voltage applied to the source of one of driver transistors34 a, 34 b, retention disturb process 104 is performed by the automatedtest equipment reducing the array power supply voltage V_(DDA) to thedesired level. For example, a typical retention disturb involvesapplying ½ the nominal power supply voltage V_(DDA) (e.g., 0.6 V wherethe nominal power supply voltage is 1.2 V). Cells 30 under test are thenstatically held at this reduced power supply voltage, for example for aduration of on the order of 100 msec, following which the power supplyvoltage is returned to those cells.

In the case of cells 30′_(jk) and 30″_(jk), the asymmetric bias appliedin process 102 and retention disturb process 104 may be performedsimultaneously, insofar as retention disturb process 104 involves thereduction of array power supply voltage. For example, asymmetric biasmay be applied (for the “0” data state) by reducing the voltages at botharray power supply nodes V_(DDAa), V_(DDAb), but with the voltage atarray power supply node V_(DDAb) being reduced more than the voltage atarray power supply voltage node V_(DDAa). This operation would reducethe bias for the “on” state load transistor 33 b (for the “0” datastate), and also reduce the array power supply voltage generally tocarry out the retention disturb.

After retention disturb process 104, normal (symmetric) bias is thenapplied to all transistors of cells 30 under test in process 106. Inprocess 107, cells 30 under test are then read to determine whether theretention disturb of process 104 under the asymmetric bias applied inprocess 102 caused loss of the stored data state (i.e., the “0” datastate written in process 100). The pass/fail results, including theaddresses of failing cells, are stored by the automated test equipmentin its memory, also in process 107.

In decision 108, the automated test equipment determines whether bothdata states have been tested. If not (decision 108 is “no”), a “1” datastate is then written into cells 30 under test in process 109, under the“normal” bias conditions applied in process 106. Processes 102, 104,106, 107 are then repeated for this opposite data state in the mannerdescribed above, except that the reduced bias will be applied to drivertransistor 34 b, load transistor 33 a, or both, depending on theparticular cell construction. For the example of cell 30 _(jk) of FIG. 4a, of course, a higher voltage will be applied at array ground voltagenode V_(SSAb) than at array ground voltage node V_(SSAa) in the repeatedinstance of process 102 for the “1” data state. Following this pass ofthe test sequence for the “1” data state (decision 108 is “yes”),retention stability test sequence 64 d is complete.

Referring back to FIG. 7, upon completion of asymmetric bias screenprocess 64, the remainder of the test program is then completed by theautomated test equipment for RAM 18. In this embodiment of theinvention, if RAM 18 includes redundant rows or columns (or both) ofSRAM cells 30 that are available to replace main array cells that failscreen 64, redundant replacement of any identified failed cells can beperformed in the manner shown in FIG. 7. The automated test equipmentdetermines, in decision 65, whether any SRAM cells 30 failed screens 64by interrogating its memory for any stored addresses of failed cells; ifthere are none (decision 65 is “pass”), memory array 20 is considered ashaving passed and is ready for further manufacture. If one or more SRAMcells 30 failed screen 64 than can be replaced by the availableredundant cells (decision 465 is “≧n fail”), memory array 20 isconsidered to have failed, and is disposed of or otherwise reworked asappropriate. If one or more, but fewer than the limit of, SRAM cells 30failed screen 64, conventional redundant replacement and mapping ofredundant cells is performed in process 66, and those newly enabledcells 30 may themselves be screened by screen 64 to ensure their abilityto withstand BTI degradation over the expected operating life. Assumingthat additional vulnerable bits are not identified (decision 67 is“pass”), memory array 20 is then ready for additional manufacture.

Following the test method shown in FIG. 7, and such other test andwafer-level processing as appropriate, integrated circuit 10 willproceed to the desired packaging and additional test stages of themanufacturing process. In the packaging of integrated circuit 10, it iscontemplated that the pads available at the wafer level may enable thehard-wiring of the separate array power supply voltage nodes V_(DDAa),V_(DDAb) and array ground voltage nodes V_(SSAa), V_(SSAb) to oneanother so that operation of RAM 18 in its system application will becarried out under normal bias conditions.

Embodiments of this invention provide numerous important benefits andadvantages over conventional memory test approaches. As described above,the ability to apply asymmetric bias voltage to transistors withinmemory cells enables the more direct screening of vulnerable cells thanis conventionally available by way of conventional “proxy” bias voltagesand other test vector conditions. As such, more direct and more robustscreening for later life threshold voltage shifts, including bothn-channel transistors due to PBTI and p-channel transistors due to NBTI,as well as for variations of operating temperature, is thus provided byembodiments of this invention.

While this invention has been described according to its embodiments, itis of course contemplated that modifications of, and alternatives to,these embodiments, such modifications and alternatives obtaining theadvantages and benefits of this invention, will be apparent to those ofordinary skill in the art having reference to this specification and itsdrawings. It is contemplated that such modifications and alternativesare within the scope of this invention as subsequently claimed herein.

What is claimed is:
 1. A method of testing memory cells arranged in rowsand columns in a memory array of an integrated circuit, each memory cellcomprising first and second cross-coupled inverters and a first passtransistor, each column of memory cells associated with a first bit linecoupled to the first pass transistor of the memory cells in the column,each row of memory cells associated with a word line coupled to gates ofthe pass transistors of the memory cells in the row; wherein, inoperation, the first and second inverters are each biased between powersupply and ground nodes; the method comprising: applying a normal biasvoltage to the first and second inverters of a selected memory cell;then writing a first data state to the selected memory cell; thenapplying, for a selected duration, an asymmetric reduced bias to thefirst and second inverters so that the voltage between the first andsecond array bias nodes of the first inverter of the selected memorycell differs from the voltage between the first and second array biasnodes of the second inverter of the selected memory cell; then applyingnormal bias voltages to the first and second inverters of the selectedmemory cell; and reading the state of the selected memory cell.
 2. Themethod of claim 1, wherein the first inverter in the selected memorycell comprises: a first load; and a first n-channel driver transistorhaving a gate, and having a source/drain path connected to the firstload at a first storage node, the first load and the source/drain pathof the first driver transistor connected in series between an arraypower supply node and a first array ground node; wherein the secondinverter in the selected memory cell comprises: a second load; and asecond n-channel driver transistor having a gate connected to the firststorage node, and having a source/drain path connected to the secondload at a second storage node, the second storage node being connectedto the gate of the first driver transistor, the second load and thesource/drain path of the second driver transistor connected in seriesbetween the array power supply node and a second array ground node;wherein the first data state corresponds to the first storage node at alow voltage and the second storage node at a high voltage; and whereinthe applying step comprises: applying a normal ground voltage to thesecond array ground node while applying a voltage above the normalground voltage to the first array ground node; and applying a reducedvoltage below a normal array power supply voltage to the array powersupply node for the selected duration.
 3. The method of claim 1, whereinthe first inverter in the selected memory cell comprises: a first load;and a first n-channel driver transistor having a gate, and having asource/drain path connected to the first load at a first storage node,the first load and the source/drain path of the first driver transistorconnected in series between a first array power supply node and an arrayground node; wherein the second inverter in the selected memory cellcomprises: a second load; and a second n-channel driver transistorhaving a gate connected to the first storage node, and having asource/drain path connected to the second load at a second storage node,the second storage node being connected to the gate of the first drivertransistor, the second load and the source/drain path of the seconddriver transistor connected in series between a second array powersupply node and the array ground node; wherein the first data statecorresponds to the first storage node at a low voltage and the secondstorage node at a high voltage; and wherein the applying step comprises:applying, for the selected duration, a first reduced array power supplyvoltage to the first array power supply node while applying a secondreduced array power supply voltage lower than the first reduced arraypower supply voltage to the second array power supply node.